1. Field of the Invention
The invention relates generally to the field of rotary tools used in drilling and completion of wellbores in the Earth. More specifically, the invention relates to rotary tools that make use of geared couplings between a driven input shaft and a driving output shaft.
2. Background Art
Drilling and completion of wellbores in the Earth, such as are used in the production of petroleum from subsurface reservoirs, includes the use of a number of types of rotary tools. A particularly important one of such rotary tools is the so-called “turbodrill.” A turbodrill is used in association with a drill string suspended from a drilling rig. The drill string is typically formed from lengths of steel pipe threaded together end to end and suspended from the rig by suitable hoisting equipment. Pumps force fluid called “drilling mud” through the interior of the drill string and out the bottom of the drill string through a drill bit, which performs the actual cutting of the rock formations. The exiting drilling mud cools the bit and lifts cuttings from the wellbore to the surface. When the drill string includes a turbodrill, the turbodrill itself is used to rotate the drill bit. The turbodrill includes one or more turbines disposed within a housing and ultimately rotationally coupled to the bit, such that the flow of drilling mud is converted to rotational energy to drive the drill bit.
Other applications for drilling mud flow-driven turbines include generation of electrical power to operate various formation characterization and/or drilling survey instruments known as measurement while drilling (“MWD”) instruments.
It is a characteristic of mud flow-driven turbines that they generate relatively low torque, but can rotate at relatively high speeds. In turbodrilling applications, it has been shown to be beneficial to provide a speed-reducing gear system between the turbine and the drill bit, such that the drill bit can be driven at relatively lower rotational speed, and at correspondingly higher torque. Uses for such “geared turbodrill” devices are described, for example, in R. Searle, et al., “Geared Turbodrilling Applications and Case Histories in the North Sea, Paper No. 90495, Society of Petroleum Engineers, Richardson, Tex. (2004).
Reduction gear systems when operated in wellbores using well fluid movement for a rotary power source have a number of limitations. First, it is necessary to provide a rotary seal between the turbine-driven shaft, which is necessarily exposed to the wellbore fluid, and the interior of the driven device. In a geared turbodrill, the device includes a planetary gear set. The interior of the device is usually filled with a lubricant, such as oil, that is subject to degradation when exposed to high temperatures (typical in wells drilled into the Earth). The fluid in the interior of the device must ordinarily be pressure compensated to be maintained at the same fluid pressure as the hydrostatic fluid pressure in the wellbore, or the rotary seal will be subjected to differential fluid pressure in excess of its capacity to exclude wellbore fluid from the interior of the device. Pressure compensation devices known in the art may be subject to delays in compensation, causing fluid penetration into the interior of the device or fluid leakage. To limit fluid intrusion caused by such delay, preferably, the pressure compensation device maintains a slightly higher fluid pressure inside the device than in the wellbore. The slight pressure differential has the effect of causing slow, but constant loss of the pressure compensating fluid. Thus, even under ideal conditions the typical rotating seal device has a finite time that it can be used in a wellbore before removal to replenish the compensating fluid.
In the case of rotary devices using gears to multiply or reduce output speed relative to input speed, loss of lubrication can lead to gear failure. Application of abrupt high torque has also been known to cause gear failure. While the strength of the gears could otherwise be increased by increasing the size of the gears, such remedy is limited in the case of wellbore tools because such tools are typically limited in diameter to that of the wellbore being drilled less an annular space to allow cuttings and return mud flow to the Earth's surface. In turbodrilling applications, as well as in wellbore drilling generally, such abrupt torque application is frequent, because of the highly variable mechanical properties of the Earth formations being drilled and the relatively low resolution control over the amount of axial force applied to the drill bit on the typical drilling rig.
Typical turbodrill reduction gear devices include planetary gear sets. Planetary gears are particularly suitable for wellbore applications because in wellbore applications the input and output shafts of the gear devices are typically coaxial. Planetary gears are generally limited to about 3¼ to 3½ to 1 input to output ratio because of the limitations of shaft and gear diameters, among other factors. To step up or step down the speed between input and output shafts more than would be feasible with a single planetary gear set would require coupling a plurality of such gear sets end to end. Such arrangement increases the overall length, weight, complexity and required lubrication reservoir capacity of the gear set.
There exists a need to have a wellbore rotary device that can include a gear unit, but does not require rotary seals or pressure compensation. There also exists a need for a rotary device for use in a wellbore that can have a wide range of gear reduction ratios without the need for compound gear sets.